Electronic Device Containing Nanowire(s), Equipped with a Transition Metal Buffer Layer, Process for Growing at Least One Nanowire, and Process for Manufacturing a Device

ABSTRACT

The electronic device comprises a substrate ( 1 ), at least one semiconductor nanowire ( 2 ) and a buffer layer ( 3 ) interposed between the substrate ( 1 ) and said nanowire ( 2 ). The buffer layer ( 3 ) is at least partly formed by a transition metal nitride layer ( 9 ) from which extends the nanowire ( 2 ), said transition metal nitride being chosen from: vanadium nitride, chromium nitride, zirconium nitride, niobium nitride, molybdenum nitride, hafnium nitride or tantalum nitride.

TECHNICAL FIELD OF THE INVENTION

The invention relates to the field of semiconductor materials and moreprecisely that of devices containing semiconductor nanowire(s).

The invention relates more particularly to a device comprising asubstrate, at least one semiconductor nanowire and a buffer layerinterposed between the substrate and said nanowire.

PRIOR ART

In the field of nanowire growth, it is known practice to use nucleationlayers such as AlN (aluminum nitride) or TIN (titanium nitride). Theselayers may be deposited directly by LPCVD (low-pressure chemical vapordeposition) or by APCVD (atmospheric-pressure chemical vapor deposition)as described in document WO 2011/162715.

This document WO 2011/162715 states that semiconductor nanowires have agrowth that may be promoted if the crystallographic orientation of acrystalline substrate enabling the growth is oriented in the direction[111] in a face-centered cubic structure of “NaCl” type, or in thedirection [0001] or along the axis “c” in a “hexagonal” structure.

If the substrate is not correctly oriented. It is possible to deposit anAlN or TIN nucleation layer whose crystallographic structure will have apredominance of orientation in the direction [0001] for AlN which has ahexagonal structure and in the direction [111] for TIN which has an fccstructure.

It results from the foregoing that the crystallographic orientation ofthe growth support for nanowires is important. The predominance in acorrect direction of a crystallographic structure should thus beoptimized in order to promote the growth of the nanowires from thiscrystallographic structure.

Moreover, the nucleation layer should not be an obstacle to thepolarization of a nanowire-based light-emitting nanodiode, for exampleperformed by injection of electrons by the substrate to the nanowire viathe nucleation layer, the nanowire constituting the N part of the PNjunction of the nanodiode. A solution must therefore be found wherebythis polarization may be optimized while at the same time conserving orproviding new advantages during the growth of the nanowires.

Object of the Invention

The aim of the present invention is to propose a device whose nanowirehas a good crystallographic orientation and advantageously allowsoptimized polarization of the nanowire.

Steps toward this aim are taken by an electronic device comprising asubstrate, at least one semiconductor nanowire and a buffer layerinterposed between the substrate and said nanowire, the buffer layerbeing formed at least partly by a transition metal nitride layer fromwhich extends the nanowire, said transition metal nitride being chosenfrom: vanadium nitride, chromium nitride, zirconium nitride, niobiumnitride, molybdenum nitride, hafnium nitride or tantalum nitride.

Advantageously, the transition metal nitride layer comprises nitrogen,the concentration of which in the stable state of the transition metalnitride varies within a range of less than or equal to 10%.

According to one implementation, the nanowire is made of galliumnitride.

Preferably, the buffer layer is electrically conducting, so as to allowan electrical contact between at least one electrically conducting partof the substrate and the nanowire.

According to one embodiment, the end of the nanowire that is remote fromthe substrate is electrically doped according to a first type, and thedevice comprises a doped electrically conducting element of a secondtype placed at the end of the nanowire that is remote from the substrateso as to form an electrical junction, especially a light-emitting diodejunction.

According to a perfection, the device comprises quantum wells placed atthe interface between the nanowire and the doped electrically conductingelement.

In addition, the device may comprise a nanowire polarization element soas to allow the generation of a light wave at said nanowire.

The invention also relates to a process for growing at least onesemiconductor nanowire comprising a step of producing, on a substrate, abuffer layer at least pertly formed by a nucleation layer for the growthof the nanowire and a step of growth of the nanowire from the nucleationlayer, the nucleation layer being formed by a layer of transition metalnitride chosen from: vanadium nitride, chromium nitride, zirconiumnitride, niobium nitride, molybdenum nitride, hafnium nitride ortantalum nitride.

According to a first embodiment, the buffer layer is deposited as avapor phase from a gas mixture comprising nitrogen and a transitionmetal chosen from vanadium, chromium, zirconium, niobium, molybdenum,hafnium or tantalum.

According to a second embodiment, the buffer layer is produced via thefollowing steps: deposition on the substrate of a layer of a transitionmetal chosen from vanadium, chromium, zirconium, niobium, molybdenum,hafnium or tantalum; nitridation of at least part of the depositedtransition metal layer so as to form the layer of transition metalnitride having a surface intended for the growth of the nanowire.

Advantageously, and in a manner that is applicable to the process ingeneral, the transition metal nitride layer is made such that, onceformed, it comprises nitrogen, the concentration of which in the stablestate of the transition metal nitride varies within a range of less thanor equal to 10%.

According to an implementation that is applicable to the secondembodiment, the nitridation step of said at least part of the transitionmetal layer is performed so as to at least partly modify thecrystallographic structure of the transition metal layer toward a facecentered cubic or hexagonal, associated with the transition metalnitride layer.

Advantageously, the nitridation step comprises: a first nitridationsubstep at least partly performed at a first temperature by imposing aninjection of a nitridation gas at a first flow rate; a secondnitridation substep at least partly performed at a second temperatureless than or equal to the first temperature by imposing an injection ofthe nitridation gas at a second flow rate different from the first flowrate.

According to a particular example, the injected nitridation gas isammonia, and the first temperature is between 1000° C. and 1050° C.,especially equal to 1050° C.; the first flow rate is between 500*V/8sccm and 2500*V/8 sccm, especially equal to 1600*V/8 sccm; the secondtemperature is between 950° C. and 1050° C., especially equal to 1000°C.; the second flow rate is between 500*V/8 sccm and 2500*V/8 sccm,especially equal to 500*V18 sccm; in which V is the total capacity inliters of a corresponding nitridation chamber.

Preferably, the nitridation step is performed in a nitridation chamberplaced at a pressure of between 50 mbar and 800 mbar, especially 100mbar.

For example, the nanowire growth step is performed after the secondnitridation substep, or is initiated during the second nitridationsubstep.

Advantageously, the nanowire growth step comprises a step of injectingGa so as to form the gallium nitride nanowire, said nanowire extendingfrom a growth surface of the nucleation layer.

According to one implementation, with the substrate being silicon, thestep of depositing the transition metal layer is configured such thatthe interdiffusion of silicon into the deposited transition metal layeris less than 10 nm and/or so as to conserve a non-silicided slice of thetransition metal layer of at least 2 nm.

If the deposited transition metal is chosen from Cr, V or Ti, saidtransition metal is preferentially deposited at a temperature below 100°C.

In the case where the substrate 1 is based on silicon, the step ofdepositing the transition metal layer 6 preferentially comprises apreliminary step of determining the thickness of the transition metallayer to be deposited, comprising: a step of determining a firstdiffusion length of the silicon into the transition metal layer duringthe future deposition of the transition metal layer as a function of thetransition metal used and of the deposition temperature; a step ofdetermining a second diffusion length of the silicon into the transitionmetal layer during the future nitridation step of the transition metallayer; said thickness of the transition metal layer to be depositedbeing dependent on the desired thickness of the transition metal nitridelayer and on a thickness of a silicided slice of transition metalobtained in the future transition metal layer from the first and seconddetermined diffusion lengths.

According to one implementation, the process may comprise a step inwhich the substrate is provided such that it has a resistivity ofbetween 1mΩ.cm and 100 mΩ.cm.

Preferably, the process comprises, before deposition of the transitionmetal layer, a step of deoxidation of a surface of the substrateintended to receive the transition metal layer.

The invention also relates to a process for manufacturing a device asdescribed, comprising a step of performing the growth process asdescribed.

The manufacturing process may advantageously comprise the followingsteps: the electrical doping of a first type of at least one end of thenanowire which is opposite the substrate; the formation of anelectrically doped element of a second type opposite the first type atthe end of the nanowire opposite the substrate.

In addition, the manufacturing process may comprise a step of formingquantum wells placed at the interface between the nanowire and theelectrically doped element of the second type.

SUMMARY DESCRIPTION OF THE DRAWINGS

Other advantages and characteristics will emerge more clearly from thedescription which follows of particular embodiments of the invention,which are given as nonlimiting examples and represented on the attacheddrawings, in which:

FIG. 1 illustrates a particular embodiment of an electronic deviceaccording to the invention,

FIG. 2 is a view in cross section of a step of formation of a nucleationlayer,

FIG. 3 is a view in cross section of a step of nucleation of at leastone nanowire from the nucleation layer,

FIG. 4 illustrates a representation of the X-ray diffraction spectrumfor identifying the types of crystallographic structures present in anNb-based transition metal layer before nitridation and afternitridation,

FIG. 5 illustrates a representation of the X-ray diffraction spectrumfor identifying the types of crystallographic structures present in anHf-based transition metal layer before nitridation and afternitridation,

FIG. 6 represents in detail an implementation of a nitridation stepaccording to one embodiment of the invention,

FIGS. 7 and 8 illustrate different steps for preparing a nucleationlayer,

DESCRIPTION OF PREFERENTIAL MODES OF THE INVENTION

The device described below differs from the prior art especially in thematerials used and intercalated between the nanowire and the substrate.

The term “microwire” or “nanowire” in the rest of the descriptionpreferentially means a three-dimensional structure of elongated shapewhose longitudinal dimension is at least equal to once the transversedimension(s), preferably at least five times and even morepreferentially at least ten times. The transverse dimension(s) arebetween 5 nm and 2.5 μm. In certain embodiments, the transversedimensions may be less than or equal to about 1 μm, preferably between100 nm and 300 nm. In certain embodiments, the height of each nanowiremay be greater than or equal to 500 nm, preferably between 1 μm and 50μm.

As illustrated in FIG. 1, the electronic device 4 comprises a substrate1, at least one semiconductor nanowire 2 and a buffer layer 3 interposedbetween the substrate 1 and said nanowire 2. The buffer layer 3 isformed at least partly by a transition metal nitride layer from whichextends the nanowire 2, said transition metal nitride being chosen from:vanadium nitride, chromium nitride, zirconium nitride, niobium nitride,molybdenum nitride, hafnium nitride or tantalum nitride. The term“extends from” means that the nanowire 2 has a defined length betweentwo longitudinal ends 2 a, 2 b, a first longitudinal end 2 a being incontact with the transition metal nitride layer and a secondlongitudinal end 2 b being remote from the transition metal nitridelayer. Preferably, the second longitudinal end 2 b is the most distantfrom the transition metal nitride layer.

The choice of this transition metal nitride layer makes it possible togive said transition metal nitride layer electrical conduction nature ofmetallic type and refractory nature of ceramic type. These properties(metallic/refractory), which are in principle antagonist, may beachieved by the transition metal nitrides mentioned above. In point offact, the refractory nature may be achieved by a material whose meltingpoint is associated with a temperature above about 1800° C., which isthe case for the transition metal nitrides targeted above In addition,such a transition metal nitride layer was advantageously able to serveas a nucleation layer during growth of the nanowire so as to obtain ananowire oriented along the axis C substantially perpendicular to thesubstrate 1. The term “substantially perpendicular to” means exactlyperpendicular or perpendicular to more or less 10°. This slightdisorientation of more or less 10° nevertheless permits the performanceof subsequent technological steps for preparing more complete devices.

Tungsten, although present in the same column of the Periodic Table aschromium and molybdenum, is set aside from the list since it hasinsufficient stability properties at high temperatures, which does notallow efficient growth of the nanowires. In other words, such a device 4cannot be obtained, or will be greatly defective, if the transitionmetal nitride layer is based on tungsten.

Moreover, titanium nitride is also set aside since, during itsdeposition, it gives rise to defects in the structure of the layer notallowing optimum growth of nanowire(s). Thus, the device 4 would havegreater probabilities of being defective.

In point of fact, the transition metal nitride layer advantageouslycomprises nitrogen, the concentration of which in the stable state ofthe transition metal nitride varies within a range of less than or equalto 10%. The term “stable state” means that the transition metal nitrideremains thermally and chemically stable irrespective of the annealingconditions The transition metal nitride may be noted M_(x)N_(y), with Mrepresentin transition metal used, N the nitrogen compound, x and yrepresenting the stoichiometric conditions of the transition metalnitride. By setting x as 1, y may vary within a certain measure while atthe same time conserving a stable state of the transition metal nitride,this variation also being known as the stoichiometric deviation andbeing noted ΔN (corresponding to the range of less than or equal to 10%targeted above). If ΔN is too large, the associated structure may beconsidered as defective. This is due to the fact that, in general, amononitride structure consists of two subnetworks: the metal atomsoccupy a first subnetwork of face-centered cubic orientation and thenitrogen atoms occupy all or part (especially as a function of thecomposition) of the octahedral sites of the metal network to form asecond but offset face-centered cubic subnetwork, especially offset bya/2, “a” representing the lattice parameter of the fcc structure. In theparticular case of titanium nitride, the composition in the stable statemay be between Ti₁N_(0.6) and Ti₁N_(1.1) (or an N concentration from37.5% to 52.4%, i.e. a ΔN of 15%) This size of ΔN means that forTi₁N_(0.6) the nitrogen subnetwork is lacunar and that for Ti₁N_(1.1)the metal subnetwork is lacunar. For comparative purposes: for vanadiumnitride, the concentration of N in the stable state ranges between 41.9%and 50%, i.e. a ΔN of 8.1%; for niobium nitride, the concentration of Nin the stable state ranges between 48% and 51.4%, i.e. a ΔN of 3.4%, fortantalum nitride, the concentration of N in the stable state rangesbetween 48% and 50%, i.e. a ΔN of 2%; for chromium nitride, theconcentration of N in the stable state ranges between 49.5% and 50%,i.e. a ΔN of 0.5%; for molybdenum nitride, the concentration of N in thestable state ranges between 29% and 35.4%, i.e. a ΔN of 6.4%; forhafnium nitride, the concentration in the stable state ranges between42.8% and 52.8%, i.e.a ΔN of 10%; for zirconium nitride, theconcentration in the stable state ranges between 40% and 50%, i.e. a ΔNof 10%.

Advantageously, the nanowire(s) 2 are made of gallium nitride Galliumnitride is a good candidate for forming an electrooptic device.Specifically, such a nanowire 2 made of gallium nitride makes itpossible to form a light nanoernitter, GaN-based quantum wells may beadded either in shell form around the nanowire, or in the continuity ofthe axis of the nanowire (axial structure). Depending on the compositionof these GaN-based quantum wells, the spectral domain of the lightemission may cover a wide wavelength range extending from ultraviolet toinfrared.

Preferably, the buffer layer 3 is electrically conductive so as to allowan electrical contact between at least an electrically conductive partof the substrate 1 and the nanowire 2. The substrate 1 in its entiretymay also be electrically conductive, and may then be made, for example,of n-doped silicon.

According to a particular embodiment, the end 2 b of the nanowire 2 thatis remote from the substrate 1 is electrically doped according to afirst type (for example of n type), and the device comprises a dopedelectrically conductive element 5 of a second type (for example of ptype) placed at the end 2 b of the nanowire 2 that is remote from thesubstrate 1 so as to form an electrical junction. This electricalfunction is preferentially a light-emitting diode junction. The element5 doped so as to form a junction with the end 2 b of the nanowire 2 mayat least partly cover the nanowire 2 at said end 2 b. Preferably, thedoped element 5 forms a sheath around the end 2 b of the nanowire 2.

Preferentially, the device 4 comprises quantum wells (not shown) placedat the interface between the nanowire 2 and the doped electricallyconductive element 5.

The device may comprise an element 100 for polarization of the nanowire2 so as to allow the generation of a light wave at said nanowire. It isthus understood that the device may be an electrooptic device. Apolarization element comprises the means necessary for generating acurrent crossing the device. In the particular example of FIG. 1, thepolarization element 100 may comprise a first electrical contact 100 ain direct contact with the doped electrically conductive element 5, asecond electrical contact 100 b in direct contact with the substrate 1and an energy source 100 c. This makes it possible, for example, toinject electrons and holes so as to bring about their recombinations ina zone Z1 of the nanowire 2.

Moreover, in the context of an electrooptic device, the quantum wellsmay form, where appropriate, confinement zones so as to increase theemission yield of the nanowire 2.

An example based on a nanowire 2 has been given: needless to say, thedevice 4 may comprise a plurality of nanowires extending from the metalnitride layer so as to form, for example, a matrix of pixels. Theelements 5 may then all be electrically linked to each other.

The device described above may be advantageously made at least partlyvia the process described below.

In general, in FIGS. 2 and 3, the process for growing at least onesemiconductor nanowire 2 comprises a step of producing, on the substrate1, the buffer layer 3 (FIG. 2) formed at least partly by a nucleationlayer for the growth of the nanowire 2 and a step of growth of thenanowire 2 (FIG. 3) from the nucleation layer. On the example of FIGS. 2and 3, the buffer layer 3 consists of the nucleation layer. Inparticular, the nucleation layer is formed by a layer of a transitionmetal nitride chosen from: vanadium nitride, chromium nitride, zirconiumnitride, niobium nitride, molybdenum nitride, hafnium nitride ortantalum nitride. In other words, it may also be considered that thenucleation layer consists of a transition metal nitride layer asdescribed. In order to optimize the growth, this nucleation/transitionmetal nitride layer may have a minimum thickness of 2 nm, and preferablybetween 2 nm and 50 nm.

This buffer layer 3 may be made via any type of deposition technique.The transition metal nitride layer (i.e. the nucleation layer) thusobtained also makes it possible, by virtue of the transition metal used,to have a smaller gap than the AlN-based nucleation layers that havebeen very commonly used to date as nucleation layer. Thus, when thesubstrate 1 is based on silicon, the buffer layer 3 according to theinvention has at its interface with the substrate 1 a potential barrierthat is easier to cross than in the case where AlN is used, this givingan advantage when it is desired to polarize one or more nanowires fromthe substrate 1.

According to a first embodiment, the buffer layer 3 is deposited as avapor phase from a gas mixture comprising nitrogen and a transitionmetal chosen from vanadium, chromium, zirconium, niobium, molybdenum,hafnium or tantalum. The nucleation layer is thus obtained directly,after this deposition. According to this first embodiment, the bufferlayer 3 may consist of the nucleation layer, which itself consists ofthe transition metal nitride layer.

According to a second embodiment, the buffer layer 3 is produced via thefollowing steps: the deposition onto the substrate 1 of a layer of atransition metal chosen from vanadium, chromium, zirconium, niobium,molybdenum, hafnium or tantalum; and the nitridation of at least part ofthe deposited transition metal layer so as to form the transition metalnitride layer having a surface intended for growing the nanowire(s).According to one implementation applicable to the different transitionmetals, especially for Hf, Nb, Ta, the deposited transition metal layermay have a thickness of between 20 nm and a few hundred nanometers (forexample 200 nm). For the other transition metals, a thickness of 20 nmwill be preferred. The deposition of the transition metal layer may beperformed by PVD (physical vapor deposition) from a metal target, forexample a continuous-current spray passing over the target. During thisstep of deposition of the transition metal, the substrate may bemaintained at room temperature. In a general manner applicablethroughout the description, the term “room temperature” means atemperature preferably between 20° C. and 50° C. The pressure in the PVDchamber during the deposition of the transition metal may be between3×10⁻³ mbar and 6×10⁻³ mbar.

After various tests, it was possible to observe that the growth of thenanowire(s) was promoted by this nucleation layer formed in two steps,it is thus understood that this second mode is preferred.

As indicated previously in the particular description of the device, andin a manner applicable to the various embodiments of the growth process,the transition metal nitride layer is made such that, once formed, itcomprises nitrogen, the concentration of which in the stable state ofthe transition metal nitride varies within a range of less than or equalto 10%.

An example has been given based on the growth of a nanowire, but thegrowth process is not limited to this sole example and makes itpossible, during the growth step, to grow a plurality of nanowires sideby side from the nucleation layer.

It is understood from the foregoing that the function of the nucleationlayer is to promote the nucleation of the nanowire(s). In addition, thisnucleation layer is preferably chosen so as to protect the substrate 1from any degradation during the growth (which may be the case if thesubstrate is made of silicon and the nanowire made of gallium nitride),and/or to conserve good stability at high temperatures (in the case oftemperatures above 500° C.), and/or to give good electrical conductionespecially when it is desired to polarize each nanowire and to injectcurrent via the substrate 1. The materials of the nucleation layertargeted above make it possible to satisfy these problems.

As regards the substrate 1, the process may, in a nonlimiting mannercomprise a step in which the substrate is provided such that it has aresistivity of between 1 mΩ.cm and 100 mΩ.cm. This resistivity isadvantageous when it is desired to polarize the nanowires as indicatedabove across the nucleation layer via the substrate 1.

In order to optimize the growth of the nanowire(s), it will thus besought to have, at the level of the nucleation layer, a crystallographicorientation adapted to the growth of the nanowires 2. The denser, i.e.the more predominant, this crystallographic orientation, the more thedensity of these nanowires 2 may be magnified.

In general, the step of growth of said at least one nanowire 2 maycomprise a step of injecting a material intended at least partly to formthe nanowire 2. In particular, this will be an injection of Ga so as toform the gallium nitride nanowire, said nanowire extending from thegrowth surface of the nucleation layer. To form the gallium nitridenanowire 2, the injection of Ga may be performed concomitantly with aninjection of NH₃ or N₂. In general, for the synthesis of GaN, it is thereaction of Ga with NH₃ and not with N₂ that is used. The injection ofGa may be performed in a chamber adapted to the growth of nanowires.

The description below applies to the second embodiment of he growthprocess, i.e. to the metal nitride layer obtained in two steps.

In the case of the second embodiment of the growth process, it turns outthat the metal nitride layer thus formed has growth sites whosecrystallographic orientation is more favorable to the growth ofnanowires than in the first mode. These growth sites are in greaternumber and have a better distribution than in the first mode.

It results from the foregoing that the crystallographic orientation ofthe nucleation layer, especially on a surface intended for growing thenanowire(s), is of importance in the context of promoting the growth ofnanowires.

Thus, the step of deposition of the transition metal layer ispreferentially performed such that said transition metal layer has,before the nitridation step, at least partly a crystallographicstructure of centered cubic (CC) type for a layer of a transition metalchosen from Cr, Mo, V, Nb or Ta, or of hexagonal type for a layer oftransition metal chosen from Zr and Hf.

The specific orientations targeted previously make it possible toperform the step of nitridation of said at least part of the transitionmetal layer so as to at least partly modify the crystallographicstructure of the transition metal layer toward a face-centered cubicstructure, which is especially oriented [111], or hexagonal, which isespecially oriented [0001] or along the direction of the axis “C”,associated with the transition metal nitride layer. The transitionmetals of columns 4 and 6 preferentially form nitrides having an feecrystallographic structure, whereas the transition metals of column 5may form nitrides having air fcc or hexagonal crystallographicstructure.

FIGS. 4 and 5 each illustrate an X-ray diffraction spectrum foridentifying the crystallographic phases or structures present. FIG. 4shows that for the curve C1 representing the crystallographic structureof the layer of transition metal of Nb type before nitridation, there isindeed a predominance of the orientation [110] of the centered cubic(cc) structure of Nb, and that for the curve C2 representative of thehexagonal crystallographic structure of the transition metal nitridelayer NbN, there is indeed a predominance of the orientation [0004] ofthe hexagonal structure and of its orientation harmonic [0008], i.e. ofsimilar orientation to [0001]. FIG. 5 shows that for the curve C3representative of the crystallographic structure of the transition metallayer of Hf type before nitridation, there is indeed a predominance ofthe orientation [0002] of the hexagonal structure, and that for thecurve C4 representative of the face centered cubic (fcc)crystallographic structure of the transition metal nitride layer HfN,there is indeed a predominance of the orientation [111] of theface-centered cubic structure. On FIGS. 3 and 4, only the peaks areimportant for visualizing the predominance, the rest of the curverepresenting a continuous background due to the experimental device andthe sample. Similar curves may be produced by a person skilled in theart for the other transition metals and the conclusions would besubstantially identical, for example for tantalum nitride, there wouldbe a predominance of the orientation [111] of the face-centered cubicstructure of tantalum nitride.

According to a particular implementation of the second mode, especiallyillustrated in FIG. 6, the nitridation step may comprise a first substepof nitridation En1 performed at least partly at a first temperature byimposing an injection of a nitridation gas at a first flow rate, and asecond substep of nitridation En2 performed at least partly at a secondtemperature less than or equal to the first temperature by imposing aninjection of the nitridation gas at a second flow rate different fromthe first flow rate. This makes it possible to optimize thecrystallographic orientation of the nucleation layer (transition metalnitride layer), it goes without saying that these two nitridationsubsteps are performed one after the other. In particular, the firstsubstep En1 makes it possible to perform a rapid nitridation and thesecond substep En2 makes it possible to perform annealing whichstabilizes the nitride phase of the transition metal. Following thesetwo substeps En1 and En2, the transition metal nitride layer ischemically and thermally stable and can act as protection for thesubstrate (in particular if this substrate is made of silicon) duringthe growth of the nanowire(s).

The injected gas may be ammonia (NH₃) or dinitrogen (N₂). NH₃ ispreferred since it makes it possible to nitride the transition metallayer more rapidly. In point of fact, in the NH₃ form, the nitridingpower is greater than for N₂. This rapid nitridation may be importantespecially it the transition metal is capable of being transformed intosilicide: this point is addressed hereinbelow.

According to a particular example, the injected nitridation gas beingammonia, the first temperature is between 1000° C. and 1850° C.,especially equal to 1850° C., the first flow rate is between 500 sccmand 2500 sccm (sccm means “standard cubic centimeters per minute”),especially equal to 1600 sccm, the second temperature is between 950° C.and 1050° C., especially equal to 1000° C., the second flow rate isbetween 500 sccm and 2500 sccm, especially equal to 500 sccm.

The flow rates mentioned above correspond to, the volume capacity of thenitridation chamber used, i.e. a total volume of gas (for exampleN₂+NH₃.) of 8 liters in the example mentioned. For a chamber of adifferent volume, the flow rates must be adapted (for example: for an18-liter chamber, the first flow rate will especially have to be equalto 4000 sccm and the second flow rate especially equal to 1280 sccm). Inother words, the first flow rate is between 500*V/8 sccm and 2500*V/8sccm, especially equal to 1600*V/8 sccm, and the second flow rate isbetween 500*V/8 sccm and 2500*V/8 sccm, especially equal to 500*V/8sccm. V is the total capacity in liters of a corresponding nitridationchamber. The term “corresponding nitridation chamber” means herein thechamber in which the nitridation of the transition metal layer isperformed.

In general, the nitridation step is performed in a nitridation chamberplaced at a pressure of between 50 mbar and 800 mbar, especially 100mbar.

FIG. 6 illustrates in a detailed manner the nitridation step byrepresenting the change in temperature as a function of the time in anitridation chamber. In a first time T1, the temperature in thenitridation chamber rises gradually, for example at 2° C./s up to a1050° C. stage. The first nitridation substep under NH₃ En1 targetedabove begins when the temperature reaches 200° C. During this firstsubstep, the NH₃ flow rate remains constant at 1600 sccm. In a secondtime T2, concomitant with at least part of the first substep, thetemperature is maintained at 1050° C. for a time of between 5 minutesand 15 minutes. In a third time T3, the first substep En1 is continuedwhile the temperature passes from 1050° C. to 1000° C. in 60 s. In afourth time T4, the temperature in the nitridation chamber is maintainedat 1000° C. for a time of between 5 minutes and 15 minutes and thesecond substep En2 is started. In a fifth time T5, the introduction ofheat into the nitridation chamber is stopped so that the temperature ofthe nitridation chamber falls until it returns to room temperature. Theduration of T5 may correspond to the inertia of the nitridation chamber.The second nitridation substep may be continued for a given time duringthe fifth time T5. The fifth time T5 may correspond to stoppage of theheating of the chamber and then to its decrease in temperature or mayalso correspond to a step of growth of the nanowires if the chamber usedfor the nitridation is also the MOCVD chamber dedicated to the synthesisof the nanowires.

According to a particular implementation, the step of growth of thenanowire 2 is performed after the second nitridation substep En2, or isinitiated during the second nitridation substep En2.

The use of Ga to form said at least one nanowire 2 is advantageous inthe sense that the face-centered cubic or hexagonal structures of thetransition metal nitride layer (and thus of the nucleation layer) arefavorable to epitaxy of GaN. Nanowires made of gallium nitride, thecrystallographic structure of which is a hexagonal structure (wurtzitestructure) oriented along the axis [0001] or the axis C of FIG. 3, maybe readily nucleated from the nucleation layer as described.Alternatively, the nanowires may also be made of ZnO, InN or SiC.

In order to achieve optimized nanowire growth, it is preferable for thetransition metal layer to be sparingly silicide-treated (silicided)Silicidation of the transition metal layer may arise, if the substrateis based on silicon, according to two cases: during the step ofdeposition of the transition metal, and/or when it is desired to nitridethe transition metal layer to delimit the nucleation layer. The firstcase may be explained in the following manner. In point of fact, at hightemperature (about 1000° C.), the formation of silicide compounds MSi₂is promoted (M being the transition metal used). Among these silicides,only silicides of transition metals from column V (VSi₂, NbSi₂, TaSi₂),plus chromium silicide (CrSW form crystallographic phases of hexagonalstructure, which are potentially advantageous (if textured along theaxis c) for the growth of gallium nitride nanowire(s). However, thedisagreement in lattice parameter “a” between these hexagonal phases andgallium nitride (3.19 Å) is so large, respectively −30%, −36%, −33% and−25% for VSi₂, NbSi₂, TaSi₂ and CrSi₂, that epitaxy of gallium nitrideis highly improbable. Typically, the lattice parameter “a” for thehexagonal compounds VSi₂, NbSi₂, TaSi₂, CrSi₂ is, respectively: 4.57 Å,4.97 Å, 4.78 Å and 4.28 Å. Thus, a subfamily may be formed from thefollowing materials: Ti, V, Cr, Nb, Ta, Mo, i.e. metals for which theinterdiffusion coefficient with Si is high, which implies rapid growthkinetics of the new MSi₂ phase. By way of example, Cr has a coefficientof interdiffusion with Si at 850° C. of 1.5×10−7 cm²/s., i,e, adiffusion length of about 11.6 μm in 15 minutes, whereas at about 100°C., this diffusion length falls to about 80 nm in 15 minutes. For thereasons mentioned above, if the deposited transition metal is chosenfrom Cr, V or Ti, it is preferentially deposited at a temperature below100° C. so as to limit the diffusion of the silicon derived from thesubstrate 1. For Nb, the Nb−Si interdiffusion length over 15 minutes is12 nm and 2 nm, for 800° C. and 700° C., respectively. Nb may thus bedeposited at high temperature up to 700-750° C. without—or with verylittle silicidation. The other materials: Zr, Hf and Ta having smallercoefficients of interdiffusion with Si than Nb, may thus be readilydeposited from room temperature up to 750° C.-800° C. at most. Excessivesilicidation would have the consequence of not making it possible laterto obtain a transition metal nitride layer of sufficient thickness. Inother words, to generalize, when the substrate is silicon, the step ofdeposition of the transition metal layer is configured such that theinterdiffusion of silicon into the deposited transition metal layer isless than 10 nm and/or so as to conserve a non-silicided slice of thetransition metal layer of at least 2 nm. In point of fact, thisnon-silicided slice is opposite the substrate 1 and is intended to formthe transition metal nitride layer. In FIG. 7, reference 6 indicates thetransition metal layer initially deposited on the substrate 1, duringthe deposition of the layer, a slice 7 of this layer 6 may be silicidedsuch that only part 8 of the layer 6 is composed of the pure transitionmetal that can serve to form, by nitridation, the nucleation layer. Inthe second case, the nitridation step may make it necessary to work at1050° C. for a few minutes. To do this, use will preferably be made asnitriding gas of NH₃, since, by virtue of its high nitriding power, thenitridation reaction rate is higher than the silicidation reaction rate.In point of fact, in the ideal case, it is sought to form during thenitridation step at least one transition metal nitride layer 9 (FIG. 8)in the deposited transition metal layer 6, the thickness of said metalnitride layer 9 advantageously being between 2 nm and 50 nm. In order tolimit the production of a new silicide compound, the nitridation stepwill be optimized, in point of fact, after the nitridation step, it isunderstood, as illustrated in FIG. 8, that the layer that it was soughtto produce by depositing a transition metal 6 may comprise a firsttransition metal silicide layer 7 obtained during the deposition of saidtransition metal, a second transition metal silicide layer 10 depositedin the continuity of the first transition metal silicide layer 7 and thenucleation layer 9 derived from the nitridation of the layer 8 of FIG.7. Optionally, it is also possible that a residual layer 11 of puretransition metal remains intercalated between layer 9 and layer 10, thisdepending at least partly on the deposited thickness of the transitionmetal layer.

It results from the explanation of the first and second cases that ifthe substrate 1 is made of silicon, a person skilled in the art will becapable of determining the thickness of the transition metal layer to bedeposited as a function of the type of transition metal to be deposited,of the temperature of deposition of the transition metal, of theduration of the transition metal deposition step, and also of theduration of the nitridation step so that it is possible to obtain alayer of transition metal nitride of a predetermined thickness. In otherwords, with the substrate 1 being based on silicon, the step ofdeposition of the transition metal layer 6 may comprise a preliminarystep of determining the thickness of the transition metal layer 6 to bedeposited, said step of determining the thickness comprising a step ofdetermining a first diffusion length of silicon into the transitionmetal layer 6 during the future deposition of the transition metal layeras a function of the transition metal used and of the depositiontemperature; a step of determining a second diffusion length of siliconinto the transition metal layer 6 during the future step of nitridationof the S transition metal layer 6. Said thickness of the transitionmetal layer 6 to be deposited being a function of the desired thicknessof the transition metal nitride layer and of a thickness of a slice oftransition metal silicide obtained in the future transition metal layer6 from the first and second determined diffusion lengths.

Preferably, the predominant crystallographic structure of the substrate1 is of orientation [100] at least at the interface between thesubstrate 1 and the transition metal layer 6. This makes it possibleespecially to reduce the manufacturing costs.

In general, the substrate 1 will advantageously be prepared before thedeposition of the transition metal layer 6. To do this, the process maycomprise, before the step of deposition of the transition metal layer 6,a step of deoxidation of a surface of the substrate 1 intended toreceive the deposit of the transition metal layer 6. More particularly,this step of deoxidation of the surface of the silicon may be performedeither chemically (HF bath) or physically (etching of the surface byapplying a bias tension to the substrate 1). This makes it possibleespecially to remove the layer of native silicon oxide (SiO₂) which isan “insulating” barrier to the injection of electrons into thenucleation layer and into the gallium nitride nanowire.

Preferentially, the growth process described above in its variousembodiments may be used in the context of forming a device as describedpreviously.

Thus, the invention may also relate to a process for manufacturing adevice, for example as described above, comprising a step ofimplementing the growth process as has been described above, especiallyin its various implementations or embodiments.

Moreover, the manufacturing process may also comprise a step ofelectrical doping of a first type of at least one end 2 b of thenanowire 2 opposite the substrate 1. This first type is, preferably,doping of n type. In addition, the process also comprises a step offorming an electrically doped element 5 of a second type opposite thefirst type at the end 2 b of the nanowire, 2 opposite the substrate 1.This second type of doping is preferentially of p type. Thus, the end 2b of the nanowire 2 and the doped element 5 associated with this end 2 bmay form a junction of a diode intended to emit light. This junction ispreferably a homojunction, i.e. the nanowire 2 and the associated dopedelement 5 are based on the same materials, for instance gallium nitride.The preparation of a heterojunction is also possible: for example, it ispossible to use ZnO in the form of an n-doped nanowire, and then to addquantum wells based on ZnO and to use the element 9 made of electricallyP-doped GaN. In point of fact, it is currently difficult to p-dope ZnO.

The manufacturing process may also comprise a step of forming quantumwells placed at the interface between the nanowire 2 and theelectrically doped element 5 of the second type.

1. An electronic device comprising a substrate, at least onesemiconductor nanowire and a buffer layer interposed between thesubstrate and said nanowire, wherein the buffer layer is at least partlyformed by a transition metal nitride layer from which extends thenanowire, said transition metal nitride being chosen from: vanadiumnitride, chromium nitride, zirconium nitride, niobium nitride,molybdenum nitride, hafnium nitride or tantalum nitride.
 2. The deviceas claimed in claim 1, wherein the transition metal nitride layer,comprises nitrogen, the concentration of which in the stable state ofthe transition metal nitride varies within a range of less than or equalto 10%.
 3. The device as claimed in claim 1, wherein the nanowire ismade of gallium nitride.
 4. The device as claimed in claim 1, whereinthe buffer layer is electrically conductive so as to allow an electricalcontact between at least one electrically conductive part of thesubstrate and the nanowire.
 5. The device as claimed in claim 1, whereinthe end of the nanowire which is remote from the substrate iselectrically doped according to a first type, and in that it comprises adoped electrically conductive element of a second type placed at the endof the nanowire which is remote from the substrate so as to form anelectric junction, especially a light-emitting diode junction.
 6. Thedevice as claimed in claim 5, wherein it comprises quantum wells placedat the interface between the nanowire and the doped electricallyconductive element.
 7. The device as claimed in claim 1, wherein itcomprises an element for polarizing the nanowire so as to allow thegeneration of a light wave at said nanowire.
 8. A process for growing atleast one semiconductor nanowire comprising a step of producing, on asubstrate, a buffer layer formed at least partly by a nucleation layerfor the growth of the nanowire and a step of growth of the nanowire fromthe nucleation layer, wherein the nucleation layer is formed by a layerof a transition metal nitride chosen from: vanadium nitride, chromiumnitride, zirconium nitride, niobium nitride, molybdenum nitride, hafniumnitride or tantalum nitride.
 9. The process as claimed in claim 8,wherein the buffer layer is deposited as a vapor phase from a gasmixture comprising nitrogen and a transition metal chosen from vanadium,chromium, zirconium, niobium, molybdenum, hafnium or tantalum.
 10. Theprocess as claimed in claim 8, wherein the buffer layer is produced viathe following steps: deposition onto the substrate of a layer of atransition metal chosen from vanadium, chromium, zirconium, niobium,molybdenum, hafnium or tantalum, nitridation of at least part of thedeposited transition metal layer, so as to form the transition metalnitride layer having a surface intended for the growth of the nanowire.11. The process as claimed in claim 8, wherein the transition metalnitride layer is made such that, one formed, it comprises nitrogen, theconcentration of which in the stable state of the transition metalnitride varies within a range of less than or equal to 10%.
 12. Theprocess as claimed in claim 10, wherein the step of nitridation of saidat least part of the transition metal layer is performed so as to atleast partly modify the crystallographic structure of the transitionmetal layer toward a face-centered cubic or hexagonal, associated withthe transition metal nitride layer.
 13. The process as claimed in claim10, wherein the nitridation step comprises: a first nitridation substepat least partly performed at a first temperature by imposing aninjection of a nitridation gas at a first flow rate, a secondnitridation substep at least partly performed at a second temperatureless than or equal to the first temperature by imposing an injection ofthe nitridation gas at a second flow rate different from the first flowrate.
 14. The process as claimed in claim 13, wherein the injectednitridation gas is ammonia, and in that: the first temperature isbetween 1000° C. and 1050° C., especially equal to 1050° C., the firstflow rate is between 500*V/8 sccm and 2500*V/8 sccm, especially equal to1600*V/8 sccm, the second temperature is between 950° C. and 1050° C.,especially equal to 1000° C., the second flow rate is between 500*V/8sccm area 2500* V/8 sccm especially equal to 500*V/8 sccm in which V isthe total capacity in liters of a corresponding nitridation chamber. 15.The process as claimed in claim 13, wherein the nitridation step isperformed in a nitridation chamber placed at a pressure of between 50mbar and 800 mbar, especially 100 mbar.
 16. The process as claimed inclaim 13, wherein the step of growth of the nanowire is performed afterthe second nitridation substep, or is initiated during the secondnitridation substep.
 17. The process as claimed in claim 8, wherein thestep of growth of the nanowire comprises a step of injection of Ga so asto form the gallium nitride nanowire, said nanowire extending from agrowth surface of the nucleation layer.
 18. The process as claimed inclaim 10, wherein, with the substrate being silicon, the step ofdepositing the transition metal layer is configured such that theinterdiffusion of silicon into the deposited transition metal layer isless than 10 nm and/or so as to conserve a non-silicided slice of thetransition metal layer of at least 2 nm.
 19. The process as claimed inclaim 10, wherein, with the deposited transition metal being chosen fromCr, V or Ti, said transitioncreta: is deposited at a temperature below100° C.
 20. The process as claimed in claim 10, wherein, with thesubstrate being based on silicon, the step of depositing the transitionmetal layer comprises a preliminary step of determining the thickness ofthe transition metal layer to be deposited, comprising: a step ofdetermining a first diffusion length of silicon into the transitionmetal layer during the future deposition of the transition metal layeras a function of the transition metal used and of the depositiontemperature, a step of determining a second diffusion length of siliconinto the transition metal layer during the future step of nitridation ofthe transition metal layer, said thickness of the transition metal layerto be deposited being dependent on the desired thickness of thetransition metal nitride layer and on a thickness of a slice oftransition metal suicide obtained in the future transition metal layerfrom the first and second determined diffusion lengths.
 21. The processas claimed in claim 8, wherein it comprises a step in which thesubstrate is provided such that it has a resistivity of between 1 mΩ.cmand 100 mΩ.cm.
 22. The process as claimed in claim 8, wherein itcomprises, before deposition of the transition metal layer, a step ofdeoxidation of a surface of the substrate intended to receive thetransition metal layer.
 23. A process for manufacturing a device asclaimed in claim 1, wherein it comprises a step of performing the growthprocess wherein at least one semiconductor nanowire comprising a step ofproducing on a substrate, a buffer layer formed at least partly by anucleation layer for the growth of the nanowire and a step of growth ofthe nanowire from the nucleation layer, wherein the nucleation layer isformed by a layer of a transition metal nitride chosen from: vanadiumnitride, chromium nitride, zirconium nitride, niobium nitride,molybdenum nitride, hafnium nitride or tantalum nitride.
 24. The processas claimed in claim 23, wherein it comprises the following steps:electrical doping of a first type of at least one end of the nanowireopposite the substrate, the formation of an electrically doped elementof a second type opposite the first type at the end of the nanowireopposite the substrate.
 25. The process as claimed in claim 24, whichcomprises a step of forming quantum wells placed at the interfacebetween the nanowire and the electrically doped element of the secondtype.